ELECTROMIGRATION-INDUCED FAILURES IN INTERCONNECTS WITH BIMODAL GRAIN-SIZE DISTRIBUTIONS

Interconnects containing bimodal grain size distributions are known to have lower median times to electromigration-induced failure (MTTF). However, the deviation in the time to failure (DTTF) in such lines has not been well characterized. We find that Al-2% Cu-0.3% Cr interconnects with bimodally distributed grain sizes have MTTF's which are more than an order of magnitude lower than lines with monomodally distributed small grain sizes. However, the DTTF's for both types of lines are similar, and in fact slightly lower for lines with bimodal structures. An activation energy of 0.85 eV was obtained both for lines with monomodal large grain structures and bimodal grain structures, suggesting that grain boundary diffusion is the controlling mechanism in both cases. A model based simply on microstructural characteristics, e.g. the distribution of the number of grain boundaries, can explain the lower MTTF's and DTTF's for lines with bimodal structures. The implications of bimodal grain size distributions on the reliability of large numbers of lines are discussed. Also, a new, convenient graphical tool for illustrating the failure rate of interconnects with lognormally distributed failure times is presented.